Gastrointestinal disorders such as inflammatory bowel diseases (IBD) and diarrhea remain a major health burden, warranting investigations aimed at better treatment options. Increasing evidence implicates a decrease in intestinal 5-HT transporter (SERT) and the consequent high luminal 5-HT levels in the pathophysiology of diarrheal and inflammatory disorders of the intestine both in humans and experimental models of inflammation or infection. As an important example, SERT KO mice exhibit diarrheal phenotype, abnormal motility and exacerbation of inflammatory responses. Therefore, it is critical to understand the regulation of SERT, a novel target for the treatment of GI disorders. However, very little is known regarding mechanisms underlying dysregulation of intestinal SERT under pathophysiological conditions. Previous studies from our laboratory and others have demonstrated that intestinal SERT activity is subject to regulation by protein kinases, protein tyrosine phosphatases and alterations at the level of gene expression. SERT has been previously shown to be inhibited by the pro-inflammatory mediators, LPS and TNF via distinct mechanisms. For example, LPS decreased SERT levels at the plasma membrane, whereas, TNF reduced SERT mRNA expression, albeit the detailed mechanisms underlying these effects are not known. Whether SERT upregulation can prove effective in counteracting SERT inhibition and alleviation of inflammatory and diarrheal phenotype is also not known. In this regard, our preliminary studies showed that SERT function is stimulated by TGF-1 by post- translational mechanisms as well as via alterations at the level of gene expression in IECs. Thus, to establish the role of intestinal SERT as a novel target of GI disorders, proposed studies will test the hypothesis that SERT function and/or expression is increased by distinct signaling, trafficking and molecular mechanisms, which can counteract the inhibition of SERT in pathophysiological states to reverse diarrhea or inflammation. Studies proposed in Specific Aim 1 will elucidate the signaling and membrane trafficking events modulating SERT in response to TGF-1 or LPS. Specific Aim 2 will elucidate the molecular mechanisms modulating SERT gene expression in response to long-term TNF or TGF-1 and investigate the cross-talk of their signaling/molecular pathways. Specific Aim 3 will critically examine whether luminal fluid accumulation (hallmark of diarrhea) caused by TNF or inflammation in SAMP/yitc mice (model of ileitis resembling Crohn's disease) is reversed by TGF-1. Furthermore, mechanisms modulating SERT in response to TNF and their reversal by TGF-1 will be examined in the native intestine utilizing wild type, SERT knock out and DNIIR mice, (lacking TGF- receptor signaling). Overall, these studies are designed to provide valuable insights into the mechanisms upregulating intestinal SERT in pathophysiological conditions, where TGF-1 signaling is defective and/or SERT expression is downregulated. Thus, these studies should establish the role of SERT as a novel therapeutic target for intervention in IBD and associated diarrhea.